The reservoir of dormant HIV strains lurking within the genomes of infected individuals may pose an even greater obstacle to developing an effective cure for the virus.

Activating immune cells, where the latent forms of the virus hide, forces the strains to turn on and spread to other cells. Doctors can use antiretroviral drugs to kill the reactivated viral strains. But not all forms of the virus turn on when the immune cells go to work.

Scientists thought these still-latent strains were defective, but new research shows the viral forms can replicate normally. The size of the latent viral reservoir capable of reactivating in HIV-infected individuals is therefore roughly 60 times larger than scientists thought, researchers suggest in a study published October 24 in Cell.